Fibrous acoustic diaphragm



July 7, 1942. v. T. FARE 2,288,832

FIBROUS ACOUSTIC DIAPHRAGM Filed Aug. 31, 1937 Zinnentor M -ZPQIZ;

attorney Patented July 7, 1942- UNITED STATES PATENT OFFICE Victor T. Par, Collingswood, N. .I.,- assignor to 1 Radio Corporation of America, a corporation of Delaware Application August 31, 1937, Serial No. 161,833

2 Claims.

This invention relates to sound radiating members, such as speaker diaphragms, and more in particular to the type formed through the process of depositing accreted accumulations of fibrous material upon perforated forms from a liquid suspension.

Heretofcre, it has been common practice to mold or' felt sound-producing diaphragms of conoid form by the well-known process of causing a, layer of fibrous material, such as wood pulp, to become deposited upon the surface of 'a suitable former. Experience has shown that, aside from various combinations of fibrous material and the additions of various proofing and sizing compounds, accreted layers so formed are all essentially the same in texture. This can easily be shown to be the case because of the simple physical laws involved in the process of felting. In aqueous suspensions of beaten fibrous material, such as wood orrag pulp, the material is held in suspension in water and is caused to be drawn by suction through a suitable perforated form. The individual fibres will naturally settle into a similar general arrangement. For more complete information relative to this type 'of diaphragm and manufacture, reference is made to Carlisle, et al., 1,901,631, and Hawley, 1,984,018 and 1,872,081.

Sound diaphrams thus produced require ex- 'applied'to the dried member to render it more acceptable as a sound radiator.

Because a diaphragm, such as is used in the conventional type of electrodynamic loudspeaker,

is called upon to perform in a manner not usually associated with ordinary paper products, it must possess qualities not generally found in ordinary P p It is accordingly an object of my invention to provide a novel acoustic diaphragm of the felted type that shall have improved acoustic characteristics and greater strength.

A further object of my invention is to provide a new method or process for producing my improved diaphragm, as well as'a novel device for carrying out the method or process.

Other objects and advantages inherent in my invention will appear from the following description of my article, process and device.

In accordance with my invention I am able to produce a felted layer of accreted fibres that plate as shown. A strut 8 is secured to the inner are highly commingled by vibrating the perforated former and/or fibres while depositing the fibres from the liquid suspension. Articles formed in this manner are possessed of greater strength and density because the individual fibres, previously distorted and disarranged by the beating process, are shifted into more compact mutual relationship while being drawn into position. In this manner, they also tend to seek a final position of rest in the accreted layer'and a member is thus produced normally free of all stresses.

.1 have found that a frequency of or vibrations per second applied to the former gives good results with ordinary pulps, such as ground wood, rag, kraft orsulphite; however, the frequency may be varied as the article and Pulp demand. I have chosen the sound-radiating diaphragm as an embodiment of my'invention. However, it is not to be limited to said article,

as anyone skilled in the art will readily recognize v the fact that it can be advantageously used in the manufacture of otherarticles.

With reference to the drawing, Figure 1 illustrates a cross-section of a perforated former for diaphragms', showing vibrating mechanism made in accordance with my invention.

Figure 2 shows a cross-sectional view, in side elevation,'of a portion of frusto-conical diaphragm made in accordance with my invention' Referring to Fig. 1, a casting l of nonferrous metal is drilled and grooved at 10 over its surface. to provide free passage for the filtrate from the pulp solution. A substantial base plate 2 of metal is drilled and tapped foran exit pipe 4. The casting I is boltedto the base surface of the casting former l asby bolts, the center boss of the strut .8 being drilled and threaded to adjustably receive a soft steel end Ia of an armature I, preferably built up of laminations at portion lb. An air gap is preferably provided at i8 between the end of the armature and the yoke H, and is adjustable by means of the screw-threaded connection between armature portion la and the strut 8. One or more nuts .20 serve to lock the armature in position on the strut. A small clearance is pref-' erably provided at 2| between the armature and upper ends of the yoke.

A solenoid winding structure 6, terminating in the connectors I, is rigidly mounted within a laminated iron field or yoke II, in turn clamped rigidly in position by a clamp 12 to the base plate 2.

A perforated former or screen 3 is removably' placed in position over the former casting I and is provided with small holes throughout its entire surface, some of which are indicated at 9. It is upon the screen 3 that the deposited layer of pulp fibre forms and from which it is later removed after drying, as a diaphragm 22.

The complete former is moved downward into the pulp suspension i3, contained in a suitable tank i4, and vacuum connection is made through pipe 4 sliding through the stuiiing box I 5, and thence to a flexible hose l6, which in turn is connected to a vacuum pump.

While I have shown an electromagnetic means of vibrating the former, it is readily understood that vibration of the required frequency and amplitude can be supplied with any one of several well-known mechanical means. Furthermore, means may be provided for superimposing vibratory impulses directly upon the flow of liquid through the former and/ or exit tube, as Hawley 1,673,047 or Hawley 1,717,775.

Again referring to Fig. 1, it is evident that when the coil 6 is energized from a source of 60- cycle current, a 120-cycle vibration is imparted to the structure The amplitude of the imparted vibration does not need to be excessive in part I and it, in turn, is transferred to the finely perforated metal screen shown at 3. -At the instant that suction is applied through the pipe 4, fibres in the solution |3 are drawn to the surface of part 3, which, because of its violent agitation, causes the fibres to settle down and seek their mostnatural positions.v Because the accretion occurs both by suction and a shifting or shaking process, a superior type of felted product is thus produced. 1

For improved accoustic fidelity, in certain cases it is preferable to have the smaller base portion of the diaphragm stifier and denser, as for the better reproduction of higher audio frequencies. With the vibration means attached to the former i in the manner shown, the smaller base area will receive a greater amplitude of vibration. The outer portion may be held rigid and maintained free of periodic vibration. The result will be a graded diaphragm as in Nicholson'Patent 1,624,- 357, issued April 12, 1927. In some cases it may be desired to make the corrugated periphery portion denser than the adjacent portions. This can readily be accomplished by arranging means to vibrate this portion locally, keeping other portions stationary, or to a greater extent than said other vibrated portions.

Although I have shown and described certain specific embodiments of my invention, 1 am fully aware that many modifications thereof are possible. In particular, I am aware that diaphragms constructed according to the precepts of my invention may have other forms than the conoidal formsdescribed herein.

My invention, therefore, i not to be restricted certain portions of said diaphragm than in others whereby to render said certain portions stiifer than said other portions.

2. An acoustic diaphragm formed of pulp fibre and wherein the fibres are arranged in compact mutual relationship and are more densely compacted in certain portions than in other portions of the diaphragm.

. VICTOR '1'. Paint. 

